1) Field of the Invention
This invention relates to a method for forming an insulating coating on a surface of a material whose surface is made of copper or a copper-based alloy, such as a wire, a stranded cable, a band, a tube, a pipe or the like (hereinafter called a "copper material").
More specifically, this invention provides a method for forming a uniform and tough, electrical insulating layer having excellent heat resistance on a surface of a copper material by anodizing the copper material in an alkaline bath as a first step and then anodizing the thus-treated copper material in an acidic bath of a hexacyanoiron complex as a second step.
2) Description of the Related Art
A variety of methods has heretofore been proposed for the formation of an electrical insulating coating layer (hereinafter simply called an "electrical insulating layer") on surfaces of various materials, including the following methods:
i) Covering or coating of an organic material: PA1 ii) Covering or coating of an inorganic material: PA1 iii) Different from the above described covering or coating of an organic material or an inorganic material, there are methods for directly forming an electrical insulating layer on a surface of a conductor. PA1 (iv) Other methods have also been proposed, in which a copper material--which is one of best conductors and has workability such as wire drawability--is made electrically insulting at a surface thereof by chemical conversion or anodization. These methods however also have problems to be described below, so that their use in actual production is inhibited. PA1 (v) In the meantime, the present inventor made a new proposal in order to overcome the above-described drawbacks of the conventional techniques. PA1 (i) anodizing the copper material in an alkaline electrolyte bath of a caustic alkali to form a thin film layer of cupric oxide on the surface of the copper material; and PA1 (ii) anodizing the copper material, which has been anodized in step (i), in an acidic electrolytic bath of a hexacyanoiron complex.
For example, Scotch.RTM. tapes (product of 3M Co., St. Paul, Minn., U.S.A.) are made of a polyester, PTFE or polyimide material and use a thermosetting silicone rubber or an acrylic adhesive. Although they have an excellent withstand voltage (dielectric strength), their heat resistance is below 200.degree. C. PA2 Proposed coverings and coatings include, for example, flexible coverings formed by firing glass fibers in combination with an organic substance rather than simply applying glass fibers; and coatings obtained by applying inorganic polymers which contain boron, silicon and/or oxygen and can be converted to ceramics when fired. These coverings and coatings are however thick and costly so that their use for electronic devices and equipment reduced in dimensions and improved in precision is unsuitable.
Incidentally, as a simple and easy method for forming a reliable, electrical insulating layer, there is a method in which 0.1-mm thick mica is applied with an adhesive and inorganic powder. This method however involves problems, for example, in coil winding or the like because the covering thus applied has poor adhesion to the substrate. A limitation is therefore imposed on its practical utility.
These methods include, for example, formation of alumite (i.e., anodic oxidation coating of aluminum) and electrolytic deposition. These methods are both applicable only to those made of an aluminum-based material. When the degree of wire drawing becomes 0.5 mm or smaller in diameter, extreme difficulties are encountered and an increase in product cost is unavoidable. These methods therefore have poor practical utility.
In such a chemical conversion as described above, a bath is prepared generally by adding a single alkali salt at a high concentration and an oxidizing agent, and a copper material to be treated is dipped at a high temperature in the bath so that a layer of cupric oxide (CuO) is formed on a surface of the copper material. This method however requires not only a long time for the chemical conversion but also a rather high cost for the reagents, and its productivity is therefore poor.
In the above-described anodization, an electrical insulating layer composed of cupric oxide (CuO) is formed on a surface of a copper material at a high current density in an alkaline solution of a high concentration in order to ensure high productivity.
This anodization involves the problem that, as cupric oxide thus formed is instantaneously redissolved even by a slightest variation in conditions (alkali concentration, current density), the process control is extremely difficult. Another serious problem of the above-mentioned anodization resides in that an anodized product must be washed thoroughly with water. When an alkali component remains on the product, large facilities, a lot of water and waste water treatment are required for the elimination of the alkali component. In view of these requirements, the anodization mentioned above is therefore considered to have poor practical utility. This water washing poses an especially serious problem when the product has a shape inconvenient for washing as in the case of a stranded cable, unavoidably resulting in extremely low productivity.
With a view toward overcoming the above-described drawbacks in the anodization of copper materials, there has been proposed an anodization method for a copper material in which plural alkaline baths are arranged in a linear pattern, the alkali concentrations of the individual baths are successively lowered in the travelling direction of the copper material, and the average anode current in each bath is lowered [Japanese Patent Application Laid-Open (Kokai) No. SHO 58-31099].
In the conventional anodization methods for copper materials including the improved anodization methods described right above, an electrical insulating layer formed on a surface of each copper material and composed of cupric oxide (CuO) has a large thickness and is weak against external strains so that it tends to develop cracks. Moreover, the heat resistance of the electrical insulating layer and its adhesion strength to the substrate are insufficient. For these reasons, the conventional anodization methods for copper materials cannot fully meet the stringent requirements for coils and the like that an extremely thin, heat-resistant, peel-free, electrical insulating layer must be surely formed.
Namely, the present inventor proposed a novel method for forming on a surface of a copper material an electrical insulating layer composed of copper oxide and copper ferri(or ferro)cyanide, which comprises anodizing the copper material in a hexacyanoiron complex bath on an acidic to neutral side which is totally different from the conventional anodization making use of an alkaline bath [Japanese Patent Application Laid-Open (Kokai) No. HEI 3-240999; U.S. Pat. No. 5,078,844; French Patent No. FR 91 01965; Australian Patent (Acceptance) No. 633,785].
The above-described anodization proposed by the present inventor and making use of a hexacyanoiron complex can attain the primary object, that is, to form a rough, electrical insulating layer on a surface of a copper material. There is however room for further improvements, since a deterioration in the electrolytic bath and resulting variations in breakdown strength of formed electrical insulating layers are observed. Described specifically, the method proposed by the present inventor still has room for further improvements in order to apply it for the production of a copper material useful as a structural material having a tough electrical insulating layer on a surface thereof.
Incidentally, as a method for forming a colored decorative coating layer, more specifically a coating layer of a light brown to brown color on a surface of a copper material, it is known to use an anodizing electrolyte composed of an aqueous solution of potassium ferrocyanide [K.sub.4 Fe(CN).sub.6 ] [SU 1216257A (UKR LOCAL), see Derwent Abstract No. 86-283986/43]. This method is however different in nature from the method of the present invention which, as will be described subsequently herein, is to form a tough electrical insulating layer having a composite layer structure of a blackish brown color.
As described above, the novel method proposed by the present inventor and featuring anodization of a copper material in an acidic electrolytic bath of a hexacyanoiron complex is required to overcome problems such as a deterioration of the electrolytic bath and variations in breakdown strength of formed electrical insulating layers